〜ておく vs 〜てしまう: Preparation vs Completion in Japanese

2026年4月23日

Two useful て-form extensions: ておく (doing something in advance) and てしまう (completing or accidentally doing). Both are common in everyday Japanese conversation.

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At a Glance

Pattern	Meaning	Nuance	Example
〜ておく	Do (sth) in advance / for later	Preparation, leaving it ready	予約しておく (book in advance)
〜てしまう	Completely do / accidentally do	Completion or regret	食べてしまった (ate it all / accidentally ate)

〜ておく — Doing Something in Advance

ておく (te + oku = to put/place) means to do something and leave the result for later use.

明日の会議のために資料を準備しておいた。 — I prepared the materials for tomorrow’s meeting (in advance).

旅行の前に、ホテルを予約しておこう。 — Let’s book the hotel in advance before the trip.

Casual spoken form: ておく → とく (e.g., 買っておく → 買っとく)

〜てしまう — Completion or Regret

てしまう has two related meanings:

1. Completion (did thoroughly/fully):

宿題をやってしまった。 — I finished all the homework. (satisfied completion)

2. Regret / accident (did something unintended):

財布を忘れてしまった。 — I went and forgot my wallet. (regret)

ケーキを全部食べてしまった。 — I ate all the cake. (regret / guilty)

Casual spoken form: てしまう → ちゃう (e.g., 食べてしまう → 食べちゃう)

Yuka

How do I know when しまう means completion vs regret?

Rei

Context! 宿題をやってしまった (done with homework = relief). お菓子を全部食べてしまった (ate ALL the snacks = regret). The feeling of the speaker — and the situation — determines which meaning applies.

Yuka

And とく vs ちゃう are just casual versions?

Rei

Right. In conversation, Japanese people often shorten: 買っておく → 買っとく, 食べてしまう → 食べちゃう. You’ll hear these contractions constantly in casual speech!

Side-by-Side Comparison

Sentence	Pattern	Meaning
水を買っておいた	ておく	I bought water (for later use/in advance)
水を全部飲んでしまった	てしまう	I drank all the water (completely / uh oh)
準備しとく (casual)	ておく	I’ll prepare (in advance)
間違えちゃった (casual)	てしまう	I made a mistake (oops)

Common Mistakes

Wrong	Correct	Note
Using ておく for accidental actions	てしまう	ておく is intentional preparation; accidents use しまう
Saying しまった! as a standalone	Correct! しまった alone = “Oh no!” / “I messed up!”	しまった is commonly used as an exclamation of regret